1. Field of the Invention
This invention relates to methods of treating waste and drainage waters comprising metal and/or metalloid ions, such as ions selected from, but not limited to, iron, copper, zinc, lead, mercury, cadmium, arsenic, barium, selenium, silver, chromium, aluminum, manganese, nickel, cobalt, uranium, and antimony. More specifically, the present invention relates to a method of treating drainage waters with high hardness and alkalinity, as well as metals and/or metalloid ions, emanating from active, inactive or abandoned mine or construction sites.
2. Description of Related Art
Acid mine drainage (AMD) is polluted water that normally contains high levels of dissolved solids such as iron, manganese, aluminum, and sulfuric acid. The contaminated water is often an orange or yellowish-orange color, indicating high levels of iron. AMD comes from pyrite or iron sulfide, a mineral associated with coal. When pyrite is disturbed, as it is during coal mining, it weathers and reacts with oxygen and water to produce sulfuric acid. Metals are dissolved in the acidic environment and, consequently, high levels of iron, aluminum, and sulfate are generally observed.
Significant quantities of AMD are associated with many former and current mining operations. The problems caused by AMD include: (a) toxicity arising from particular metal contaminants, environmental pollution where the waters containing such metals are allowed to flow to areas producing contamination with toxic elements being labile or only loosely bound by adsorption; (b) the corrosive effects arising from the acidic pH values from sulfide oxidation to sulfuric acid; and (c) the lost opportunity for using the water for irrigation or human purposes.
A wide variety of methods have been proposed for remediating AMD. For example, In-situ mitigation is a method whereby limestone placements are put down to collect surface run-off and funnel it into waste rock dumps. Acidic material is capped with an impermeable layer to divert water from the acid cores. This method relies on the existence of sufficient rainfall to produce seepage or drainage that continually contacts the limestone. The method has limited efficiency for remediation and is weather dependent.
Wetlands based treatment has also been considered. This method uses a man-made wetland ecosystem to remove heavy metals. The efficiency of this method is typically less than that observed with chemical precipitation processes.
Since the passage of the Surface Mining Control and Reclamation Act (SMCRA) in 1977, coal mine operators have been required to meet environmental land reclamation performance standards established by federal and state regulatory programs. Operators must also meet water quality standards established in the Clean Water Act of 1972 (CWA), which regulates discharges into waters of the United States. Control of AMD is a requirement imposed on operators by both SMCRA and CWA. In addition to the surface mining permit, each mining operation must be issued a National Pollutant Discharge Elimination System (NPDES) permit under CWA. Allowable pollutant discharge levels are usually determined by the United States Environmental Protection Agency's (EPA) technology-based standards, or the discharge levels may be based on the more stringent water quality-based standards where discharges are being released into streams with designated uses. If AMD problems develop during mining or after reclamation, a plan to treat the discharge must be developed. Treatment of AMD includes neutralization of acidity and precipitation of metal ions to meet the relevant effluent limits.
Additionally, states have begun to regulate mine drainage. For example, in West Virginia, W. Va. Code, § 22-3-13(b)(10) provides that surface miners are required at a minimum to avoid acid or other toxic mine drainage by such measures as (i) preventing or removing water from contact with toxic producing deposits; (ii) treating drainage to reduce toxic content which adversely affects downstream water upon being released to water courses; and (iii) casing, sealing, or otherwise managing boreholes, shafts, and wells, and keeping acid or other toxic drainage from entering ground and surface waters.
In the precipitation process, enough alkalinity must be added to raise water pH so dissolved metals in the water will form insoluble metal hydroxides and settle out of the water. The pH required to precipitate most metals from water ranges from pH 6 to 9 (except ferric iron, which precipitates at about pH 3.5). The types and amounts of metals in the water, therefore, heavily influence the selection of an AMD treatment system. Ferrous iron converts to a solid bluish-green ferrous hydroxide at a pH greater than 8.5. In the presence of oxygen, ferrous iron oxidizes to ferric iron, and ferric hydroxide forms a yellowish-orange solid (commonly called yellow boy), which precipitates at a pH greater than 3.5. In oxygen-poor AMD where iron is primarily in the ferrous form, enough alkalinity must be added to raise the solution pH to 8.5 before ferrous hydroxide precipitates. In general, higher pH, especially in the range of pH 8-10, accelerates the development of the hydroxide precipitant.
A more efficient way of treating high ferrous AMD is to aerate the water in conjunction with adding a neutralizing chemical to raise the pH to between 7.5 and 8.5, causing the iron to convert from ferrous to ferric, and then to form ferric hydroxide. Aeration of the process water before and after treatment usually reduces the amount of neutralizing reagent necessary to precipitate iron from AMD. Aluminum (Al) hydroxide generally precipitates at a pH greater than 5 but also enters solution again at a pH of 9. Manganese (Mn) precipitation is variable due to its many oxidation states, but will generally precipitate at a pH of 9 to 10. Sometimes, however, a pH of 10.5 is necessary for complete removal of manganese. Interactions among metals also influence the rate and degree to which metals precipitate. For example, iron precipitation will largely remove manganese from the water at pH 8 due to co-precipitation, but only if the iron concentration in the water is much greater than the manganese content (about 4 times more or greater). If the iron concentration in the AMD is less than four times the manganese content, manganese may not be removed by co-precipitation, and a solution pH of greater than 9 is necessary to remove the manganese.
Aeration is the process of introducing air into water. Oxidation occurs when oxygen in air combines with metals in the water. If the water is oxygenated, metals generally will precipitate at lower pH values. For this reason, aeration of water can be a limiting factor in many water treatment systems. If aeration and oxidation were incorporated or improved in the treatment system, chemical treatment efficiency would increase, and costs could be reduced. Oxidants are sometimes used to aid in the completion of the oxidation process to enhance metal hydroxide precipitation and reduce metal floc volume. Chemical oxidants such as hypochlorite, hydrogen peroxide, calcium peroxide, and potassium permanganate can be used to treat mine drainage and are demonstrated effective oxidation agents.
Flocculants, inorganic coagulants, and/or organic coagulants may be used to treat AMD. They tend to increase particle settling efficiency. Coagulants reduce the net electrical repulsive forces at particle surfaces, thereby promoting consolidation of small particles into larger particles. Also, inorganic coagulants provide additional solids for enhanced particle/particle contact. The most common inorganic coagulants used in water treatment are aluminum sulfate (alum), aluminum chloride, polyaluminum chloride (PAC), aluminum chlorohydrate, ferric chloride, and ferric sulfate. The most common organic coagulants are homopolymers of diallyl dimethyl ammonium chloride, polyamines, and quaternized polyepichlorhydrins. Also, inorganic/organic coagulant blends have been used. Flocculation aggregates or combines particles by bridging the space between particles with high molecular weight chemicals. Anionic polymers dissolve to possess negatively-charged ions and the reverse occurs with cationic flocculants. Polyampholytes, when dissolved in water, possess both positively- and negatively charged ions.
After chemical treatment, the treated water flows into sedimentation ponds so metal hydroxide precipitates in the water can settle. Dissolved metals precipitate from AMD as a loose, open-structured mass of tiny grains called “floc.” Sufficient residence time, which is dictated by pond size and depth, is important for adequate metal precipitation. The amount of metal floe generated by AMD neutralization depends on the quality and quantity of water being treated which, in turn, determines how often the ponds must be cleaned. The most important physical property is a floc's settleability, which includes both the settling rate and final floe volume. Typically, calcium hydroxide and sodium carbonate produce a granular, dense floe versus a more gelatinous, loose floe generated by sodium hydroxide and ammonia. The chemical compositions of floes are typically hydrated ferrous or ferric oxyhydroxides, gypsum, hydrated aluminum oxides, calcium carbonate, and bicarbonate, with trace amounts of silica, phosphate, manganese, copper, and zinc.
In some cases, mine pool water may already contain high levels of alkalinity in addition to heavy metals. In this situation, the pH of the drainage water is closer to neutral, on the order of 5 to 7, typically about 6.5. Drainage waters of this type also typically have hardness levels expressed as calcium carbonate in excess of 100, and alkalinity levels expressed as calcium carbonate in excess of 100. Generally, under the prevailing conditions, a significant amount of CO2 is dissolved in the mine water. Iron levels are typically in excess of 0.5 ppm and predominately in the ferrous state. This type of drainage water is referred to herein as hard drainage water (HDW).
When HDW is exposed to typical AMD treatment schemes, the high pH exposure creates undesirable side effects. Typically, water that contains alkaline earth metal cations, such as calcium, barium, magnesium, etc., and several anions, such as bicarbonate, carbonate, sulfate, oxalate, phosphate, silicate, fluoride, etc., in combinations which exceed the solubility of their reaction products forms precipitates until these product solubility concentrations are no longer exceeded. For example, when the concentrations of calcium ion and carbonate ion exceed the solubility of the calcium carbonate reaction products, a solid phase of calcium carbonate will form.
Solubility product concentrations are exceeded for various reasons, such as partial evaporation of the water phase, an increase in pH, or temperature, and the introduction of additional ions, which form insoluble compounds with the ions already present in solution. For example, mine pool water that is pumped to the surface undergoes degassing of CO2 followed by an increase in solution pH. The corresponding Langelier Saturation Index (LSI) shifts from a negative value (corrosive) to a positive value (scaling). LSI is often used by water treatment specialists to describe the scaling potential of a water for applications such as, for example, in cooling towers.
The Langelier Saturation index (LSI) is an equilibrium model derived from the theoretical concept of saturation and provides an indicator of the degree of saturation of water with respect to calcium carbonate. It can be shown that the Langelier saturation index (LSI) can be correlated to the base 10 logarithm of the calcite saturation level. The Langelier saturation level approaches the concept of saturation using pH as a main variable. The LSI can be interpreted as the pH change required to bring water to equilibrium.
Water with a LSI of +1.0 is one pH unit above saturation. Reducing the pH by 1 unit will bring the water into equilibrium. This occurs because the portion of alkalinity present as CO32− decreases as the pH decreases, according to the equilibriums describing the dissociation of carbonic acid:H2CO3⇄HCO3−+H+HCO3−⇄CO32−+H+
If the LSI is negative, there is little to no potential for scale to form; the water will typically dissolve CaCO3. If the LSI is positive, scale will typically form and CaCO3 precipitation typically will occur. If the LSI is close to zero, a borderline condition for scale potential arises. Changes in water quality, pH, and/or temperature, or evaporation could change the index, leading to scale formation. The LSI is a widely used indicator of cooling water scale potential. It is an equilibrium index and deals with the thermodynamic driving force for calcium carbonate scale formation and growth. It indicates the driving force for scale formation and growth in terms of pH as a master variable.
In order to calculate the LSI, it is necessary to know the alkalinity (mg/l as CaCO3), the calcium hardness (mg/l Ca2+ as CaCO3), the total dissolved solids (mg/l TDS), the actual pH, and the temperature of the water (° C.). If TDS is unknown, but conductivity is, TDS as mg/l can be estimated. LSI is defined as:
 LSI=pH−pHs
Where:pH is the measured water pHpHs is the pH at saturation in calcite or calcium carbonate, and is defined as:pHs=(9.3+A+B)−(C+D)Where:A=(Log10[TDS]−1)/10B=−13.12×Log10(° C.+273)+34.55C=Log10[Ca2+ as CaCO3]−0.4D=Log10[alkalinity as CaCO3]
More specifically, the LSI is an equilibrium model derived from the theoretical concept of saturation, and provides an indicator of the degree of saturation of calcium carbonate for a given set of conditions. Because LSI approaches the concept of saturation using pH as a main variable, the LSI can be interpreted as the pH change required to bring water to equilibrium.
HDW typically has a slight scale forming tendency, but when HDW is exposed to typical AMD treatments, the scale forming tendency may increase dramatically as indicated by a higher LSI. In this situation, hardness in the form of calcium and magnesium ions combines with anions, such as carbonate and sulfate, to form a solid. This solid material will form scale and may combine with other chemicals in the water. The higher the hardness, the greater the amount and the faster scale will form. As the various reaction products form, they adhere to surfaces contacting the water carrying system and create deposits of scale. Also, accumulation of scale may interfere with fluid flow, facilitates corrosive processes, and/or harbors bacteria. In creeks, streams, spillways, and other waterways, the unwanted deposits can interfere with the life cycle of aquatic life. The build-up of scale is an expensive problem, causing delays and shutdowns for cleaning and removal. For example, calcium sulfate scale can be particularly difficult to remove and may be particularly aggressive toward metal substrates.
Consequently, there is a demonstrated need in the art for a method of removing metal ions from HDW where the formation of detrimental scale forming salts is minimized or eliminated.